The present invention is directed to high-capacity, high-efficiency alternating current (AC) overhead transmission lines. In one embodiment, a power transmission line with a three-phase compact delta configuration is suspended by a single crossarm. The present invention relates to a novel transmission line to maximize load-carrying ability, environmental compatibility, cost effectiveness, and public acceptance.
Public interest in clean, reliable power supplies, combined with renewable generation projects being developed in areas remote from load centers, demands transmission infrastructure capable of delivering efficiently large blocks of power over long distances. In view of the public opposition to overhead transmission in general, and 765 kilovolt (kV) (i.e., the highest transmission voltage class in the U.S.) in particular, electric utilities resort to building conventional 345 kV lines and augmenting such lines with series compensation to achieve the performance characteristics of higher-voltage transmission.
The transmission line design of the preferred embodiment boosts the performance of 345 kV lines beyond their traditional capabilities without relying on costly external devices, such as series capacitors. In the preferred embodiment, low-profile, aesthetic features minimize the environmental impact and structure costs, seeking to improve public acceptance of new transmission projects.
It has been established through engineering analysis and practice that load-carrying ability, or loadability, of a transmission line is limited by one or more of the following factors: (i) thermal rating, (ii) voltage-drop constraint, and (iii) steady-state stability limitation. Thermal rating is an outcome of the conductor and/or terminal equipment selection process, and is most limiting for lines shorter than 50 miles. Longer lines are limited primarily by voltage-drop and/or stability considerations, both of which are directly affected by the length-dependent impedance of the line.
For a given line length, the most effective method of reducing impedance and thereby improving loadability, is to raise the transmission voltage class. However, due to public opposition, multiple lower-voltage lines are built with series compensation to reduce the impedance and achieve the required loadability objectives.
Series compensation, traditionally, has been used as a near-term remedy to stretch the AC system capability. Also, in some areas, series-compensated lines serve as a substitute for higher-voltage transmission to transport sizable power blocks point-to-point over long distances. These applications invariably are accompanied by concerns such as subsynchronous resonance (SSR) and subsynchronous control interactions (SSCI), known to pose risks to electrical machinery and grid stability.
Other concerns include system protection complexities, maintenance and spare equipment requirements, electrical losses, limited life expectancy relative to that of the line itself, and future grid expandability challenges. Grid expandability is of particular concern when tapping the series-compensated line to serve a new load center or to integrate a new generating source because these developments: (i) can result in overcompensated line segments, and (ii) may be beyond the utility's control.
The new transmission line design, a 345 kV line in the preferred embodiment, minimizes these concerns while inherently offering the requisite capacity and efficiency for both long- and short-distance bulk power deliveries within the electrical grid.